This invention relates in general to the processing of wood products, and in particular to a process for recovering acetic acid from wood extracts.
Wood chips or lignocellulosic biomass are pretreated with high temperature steam/water or aqueous solutions to release sugars for fermentation purposes. In addition to sugars, compounds such as acetic acid/acetate and other organic acids or their conjugate base are also released. Because acetic acid is released in high quantities, and is both an inhibitor in fermentation processes of the sugars and has significant economic value as commodity chemical, it is advantageous to recover acetic acid from these solutions.
The use of trioctylphosphine oxide (TOPO), an extractant for removal of acids from aqueous solutions, is well known. Because of its high hydrogen bonding acceptor basisity, TOPO complexes strongly with a carboxylic acid when dissolved in water immiscible organic diluents such as undecane. In addition, TOPO is characterized by its excellent stability, high boiling point and low solubility in water.
The recovery of acetic acid from the evaporator condensate generated as part of multiple effect evaporation of acid sulfite waste pulping liquor using extraction by TOPO-undecane is practiced by Lenzing in Austria. In the Lenzing process, acetic acid and furfural are recovered at approximately 90%, further purified by distillation and then sold as final products.
Unlike the evaporator condensate used in the Lenzing process, wood extracts from wood chips or lignocellulosic biomass as described above contain dissolved sugars, uronic acids, formic acid or other organic acids. It would be expected that the acids would bind with TOPO, and as a result lead to deactivation of the extractant because the uronic and organic acids (except formic acid) are much less volatile than acetic acid and thus accumulate in the TOPO-undecane phase. Thus, it would be expected that a process similar to the Lenzing process would be unsuitable for the recovery of acetic acid from these wood extracts.